Reference is made to the patent application filed under application number DE 10 2014 222 855.7, the disclosure of which is hereby incorporated herein by reference, in its entirety, into the present patent application.
In medical examinations, for example in computed tomography scans, conducted with the aid of X-rays, X-ray detectors are used as radiation detectors. These X-ray detectors can be embodied as scintillator detectors or detectors having direct converters. What is to be understood in the following as a radiation detector is any type of detector that detects radioactive radiation, though in particular captures X-rays or other hard rays, such as gamma rays, for example.
Detectors having direct converters comprise semiconductor materials which bring about a direct conversion of the radiation incident thereon into an electrical signal. The incident X-ray radiation directly generates charge carriers in the form of electron hole pairs. As a result of a voltage (bias voltage) being applied to the semiconductor material, the charge carrier pairs are separated by the electrical field generated thereby and make their way to electrical contacts or electrodes that are mounted on the semiconductor material (see FIG. 1). This causes an electrical charge pulse to be generated which is proportional to the absorbed energy and which is evaluated by a readout electronics circuit connected downstream. Semiconductor detectors utilized in the field of human medical imaging, based, for example, on CdTe or CdZnTe, have the advantage over the scintillator detectors currently in common use there that they allow counting classified according to energy level, i.e. the detected X-ray quanta can be separated as a function of their energy into, for example, two classes (high-energy and low-energy) or into a number of classes.
In the operation of semiconducting, direct-converting radiation detectors, such as CdTe- or CZT-based detectors, for example, the phenomenon of polarization occurs under irradiation by means of gamma and X-ray radiation in particular at high intensities. This expresses itself in an unwanted change in the internal electrical field in the semiconductor material of the detector. The polarization causes changes in the charge carrier transport properties, and consequently also in the detector characteristics. In particular, the cited changes lead to a change in the signal characteristics of the measured signal as a function of time. To put it another way, the polarization induces a change in the intensity of the measured signal over time at a radiation dose that remains constant. This phenomenon is also called signal drift. In computed tomography, signal drift of the detectors leads to undesirable ring artifacts. A detector is constructed from a plurality of pixels. Because the signal drift of the individual pixels is different, the detector is subject to a distribution of the signal drift factors assigned to the individual pixels. This distribution changes with time or, as the case may be, under irradiation, the width of the distribution of the signal drift factors increasing considerably more strongly than the average value of the distribution.
One possibility of reducing the signal drift resides in exploiting the fact that the width of the distribution of the signal drift factors grows more strongly than the average value of the distribution changes. In this case a plurality of detectors are combined to form groups of individual pixels, such groups being called macropixels. The macropixels can comprise, for example, a number of 2×2, 3×3 or 4×4 individual pixels. In order to reduce the signal drift, individual pixels exhibiting severe drift are excluded completely from the signal transmission. An improved drift behavior of the detector signal is achieved in this way. However, this improvement comes at the cost of quite a high deterioration in detector efficiency, i.e. a signal intensity reduced by 5% to 20% and consequently also a correspondingly degraded signal-to-noise ratio.
One approach by which the reduction in dose efficiency can be minimized is described in the application filed under application number DE 10 2014 222 855.7. With the approach, individual pixels of a macropixel that exhibit a severe drift are weighted more weakly than individual pixels exhibiting less severe drift. In order to reduce the loss in signal intensity, a function characterizing the reduction in dose efficiency is minimized in accordance with the weightings of the individual pixels. Even with this approach, however, a certain loss in detector efficiency remains, which leads to an increase in image noise in computed tomography applications.